This invention relates to a method and apparatus for decontaminating media such as soil or chemical production wastes containing toxic chlorinated dibenzo-p-dioxins.
Chlorinated dibenzo-p-dioxins are a family of polycyclic aromatic compounds which consist of seventy-five unique congeners, which differ in their extent and sites of chlorination. The 2,3,7,8 tetrachlorodibenzo-p-dioxin congener, (2,3,7,8 TCDD), which is commonly and incorrectly referred to as dioxin, is of special environmental concern because it is one of the most toxic substances known to man. 2,3,7,8 TCDD can be acutely toxic to various mammals in single doses as low as a few micrograms per kilogram of body weight. There is a marked variation in species sensitivity towards 2,3,7,8-TCDD, for which the LD50 ranged from 0.6 ug/kg in the guinea pig to over 3,000 ug/kg in hamsters. In addition to being acutely toxic, lower doses of 2,3,7,8-TCDD can induce a variety of adverse responses including rapid weight loss, skin disorders, immune suppression, liver disease, porphyria and reproductive dysfunction. 2,3,7,8 TCDD has also been demonstrated to be highly teratogenic and is an extremely potent carcinogen.
Not all dioxins are capable of inducing toxic effects. Their toxic potential is related to the number and placement of chlorine atoms on the basic dioxin molecule. The structural requirement for toxicity appears to be substitution by chlorine in at least three of the four lateral positions (2,3,7,8) and the absence of chlorine at the non-lateral or "peri" (1,4,6,9) positions. A dramatic decrease in toxicity is observed as a result of any deviation from the 2,3,7,8 configuration. For the purposes of this invention, the dibenzo-p-dioxins substituted by chlorine in at least three of the four lateral positions shall be termed toxic chlorinated dibenzo-p-dioxins or toxic dioxins.
Dioxins are not produced for any commercial purpose, but are formed as unwanted contaminants during the synthesis and use of ortho-chlorinated phenols. They are also produced during the combustion of chlorinated organics and municipal refuse. There have been a number of industrial accidents where chlorophenol reactors have overheated and produced large amounts of dioxins. The most recent dioxin accident of this type occurred in 1976 in Seveso, Italy where an estimated 70 kg of 2,3,7,8-TCDD was released into an area populated by humans.
Although modern chlorophenol production processes limit the formation of dioxins, early techniques yielded significant amounts of dioxin by products. The herbicide 2,4,5-T and the germicide hexachlorophene, which utilized 2,4,5-trichlorophenol as an intermediate, often contained 2,3,7,8-TCDD. Technical grade pentachlorophenol has also been known to contain dioxins. Although it does not contain significant quantities of 2,3,7,8-TCDD, it has been shown to induce toxicity of the dioxin type, presumably due to other active dioxin congeners.
Dioxins and related compounds such as chlorinated dibenzofurans are also produced during the incineration of municipal wastes and possibly from the combustion of fossil fuels. Fly ash derived from municipal wastes typically contain 2,3,7,8-TCDD in the low part per billion range. Although dioxin yields are minor when compared to the levels produced during chlorophenol synthesis, combustion may represent a major source of dioxins due to the large volume of incineration and energy production currently taking place. Because of the potential toxicity of dioxins and dibenzofurans, refuse incinerators may soon be required to monitor the dioxin content of fly ash and airborne emissions and dispose of recovered particulates in an environmentally acceptable fashion.
Probably one of the most severe dioxin contamination problems emerging today is the discovery of sites in various portions of the country where chlorophenol production wastes have been disposed in an environmentally unsound manner. Chlorinated phenols have been and remain very important chemical intermediates. However, their process wastes such as sludges and reactor still bottoms can contain high levels of dioxins. Over the years, these wastes have been disposed of by various means including incineration, landfilling and even application to rural roads and fields as a means of dust control.
It is an object of this invention to provide a safe and effective means of treating toxic chlorinated dibenzo-p-dioxin-contaminated media, e.g., soil and chemical production wastes, to remove substantially all of such toxic dioxins from said media.
It is a further object of this invention to provide a device for on-site treatment of toxic chlorinated dibenzo-p-dioxin-contaminated media.
These and other objects of the invention will become apparent from the following description.